Passerine birds learn the vocalizations typical of their species during a restricted ("critical") period of development, and vocal behavior is controlled by an interconnected system of discrete brain regions. Vocal learning in birds exhibits striking parallels to vocal learning in humans: Both birds and humans exhibit perceptual sensitivity to the species-typical sounds used for vocal communication very early in life; both require exposure to adult vocal pattern as well as auditory feedback of self-produced vocalizations for normal vocal development; both exhibit gradual differentiation in terms of vocal production; finally, both birds and humans exhibit neural lateralization -- greater disruption of vocal behavior as a result of left rather than right-hemisphere damage. A series of experiments is proposed to investigate neural mechanisms of vocal learning in passerine birds. The results will have immediate application to mechanisms of speech perception and speech production in humans. 1. Using amino-acid autoradiographic techniques, we will define the complete neuroanatomical projections of afferent fibers from the vocal organ of birds (the syrinx). 2. We will utilize this information to study the effects of vocal learning of selectively eliminating these afferent fibers at various stages of development -- results of these experiments will shed light on the role of proprioceptive feedback from vocal muscles in vocal learning. 3. We will study the role of motor learning in vocal development by preventing the onset of vocalizations at the time when they normally begin, and observing effects on neural and behavioral development. 4. We will examine the normal ontogeny of the neural structures underlying vocal development, and of their projection; this information will be used as a basis for comparison with birds that undergo various experiential manipulations (such as auditory deprivation) during early development. 5. We wil study the relationship between vocal perception and vocal production by directly measuring both perceptual and productive abilities in individual animals at various stages of vocal development; results of these experiments will have implications for theories of speech perception in humans.